The present invention relates to layered heat-shrinkable films, particularly to layered heat-shrinkable films suitable for label use. More particularly, it relates to layered heat-shrinkable films for label use, which films cause only rare occurrence of wrinkles, shrinkage spots and strains by heat shrinkage, and in which films high compressive strength is compatible with bonding properties when non-halogen solvents are used.
For heat-shrinkable films, particularly heat-shrinkable films for use as labels to be fitted on the barrels of PET bottles, there have been mainly used, for example, films made of polyvinyl chloride or polystyrene. However, polyvinyl chloride has recently raised a problem that chlorine-containing gases will be evolved in the incineration for disposal, and polystyrene has a problem that printing is difficult. Further, in the collection and recycling of PET bottles, labels made of resins other than PET, such as polyvinyl chloride or polystyrene, should be separated. Therefore, heat-shrinkable films made of polyesters having no such problems have attracted considerable attention.
However, most of the heat-shrinkable polyester films rapidly shrink, so that wrinkles, shrinkage spots and strains will be left after shrinkage and the films will easily be broken by impact from outside after shrinkage. For these reasons, they are not satisfactory as shrinkable films for labels.
To avoid part of such drawbacks, JP-B 7-77757 discloses a method for improving shrinkage finish by making remarkably small breaking strength in a direction perpendicular to the main shrinkage direction.
Further, JP-A 58-64958 discloses a method for improving shrinkage finish by making small orientation return stress.
However, the films obtained by these methods cannot exhibit sufficient shrinkage finish for use on small PET bottles, which should be allowed to pass through a shrinkage tunnel for a short time, and therefore, they are also not satisfactory as shrinkable films. More specifically, when cylindrical labels formed from the shrinkable films are fitted on PET bottles and then allowed to shrunk by heat treatment, the labels may cause occurrence of wrinkles, shrinkage spots and strains by shrinkage.
Further, the production line for filling beverages in PET bottles has been accelerated, and therefore, heat-shrinkable labels have been required to have suitability for high-speed fitting, in addition to, of course, good shrinkage finish. In other words, as shown in FIGS. 1 and 2, when PET bottle 1 is fitted with label 2 at a high speed with compression member 3, the label may be bent to cause fitting failure, if the label has poor suitability for high-speed fitting. The fitting suitability of labels mainly depends on the stiffness of base films and is achieved by an increase in film thickness, which is, however, attended by harmful effects. For example, an increase in film thickness leads to an increase in film weight, which deteriorates handling properties, and to an increase in production cost.
In addition, for bonding solvents used in the formation of films into a tube shape, halogen solvents have hitherto been widely used; however, there have been increasing cases in which non-halogen solvents are used from the viewpoint of safety and health. Non-halogen solvents have a tendency to decrease the bonding properties of films as compared with halogen solvents. If the solvent bonding properties of films is improved, some problems may be raised, for example, that the stiffness of films will be decreased.
The present invention, which can solve the above problems, has been made to provide layered heat-shrinkable films which have excellent suitability for high-speed fitting and exhibit excellent shrinkage finish and which can be bonded with non-halogen solvents, without being made thicker as compared with the conventional heat-shrinkable films, in all applications including labels for small PET bottles.
Thus the present invention provides layered heat-shrinkable films, each having at least three layers, or both outer layers and an intermediate layer, wherein the heat shrinkability in the main shrinkage direction of the film is 20% or higher after treatment in hot water at 70xc2x0 C. for 5 seconds and 35% to 55% after treatment in hot water at 75xc2x0 C. for 5 seconds and 50% to 60% after treatment in hot water at 80xc2x0 C. for 5 seconds and the compressive strength of a label formed from the film meets the following formula (1):
Y greater than X2.2xe2x80x83xe2x80x83(1)
where Y is compressive strength (mN) and X is film thickness (xcexcm).
In a preferred embodiment, the glass transition temperature of said both outer layers is lower than that of said intermediate layer.
In a more preferred embodiment, said both outer layers are made of a polyester resin containing isophthalic acid at 13 mol % or higher and adipic acid at 2.6 mol % or higher in all acid components and butanediol at 10 mol % or higher in all glycol components and said intermediate layer is made of a polyester resin containing isophthalic acid at 13 mol % or higher and adipic acid at lower than 2.1 mol % in all acid components and butanediol at lower than 9 mol % in all glycol components.
In a preferred embodiment, the bonding strength when said both outer layers of the film are bonded together with a solvent is 4.5 N/15 mm or higher.
The present invention further provides heat-shrinkable labels made of the above layered heat-shrinkable films.